Abstract

We have demonstrated a direct frequency comparison between two Sr87 lattice clocks operated in intercontinentally separated laboratories in real time. Two-way satellite time and frequency transfer technique, based on the carrier-phase, was employed for a direct comparison, with a baseline of 9000 km between Japan and Germany. A frequency comparison was achieved for 83,640 s, resulting in a fractional difference of (1.1±1.6)×10−15, where the statistical part is the largest contributor to the uncertainty. This measurement directly confirms the agreement of the two optical frequency standards on an intercontinental scale.

Figures (5)

Schematic of the frequency comparison of two Sr87 lattice clocks at NICT and PTB. Two-way satellite-based comparison using the carrier-phase measures the frequency difference of two local H-masers, through which the differential frequency of two clocks is derived. pps, pulse per second; TWCP, two-way carrier-phase (time and frequency transfer).

Record of the fractional frequency difference of one typical experimental day. Corrections of systematic shifts of the atomic clocks, as well as the compensation of ionospheric delays, are included. Each point is the result of 60 s signal integration.

Allan deviation of the fractional frequency difference versus the averaging time. The instabilities shown as filled circles are derived from the latter part of Fig. 2, which contains 30,900 s of continuous data. The red solid and red dashed lines show the weighted fit and its extrapolation. Open circles are obtained by treating the whole 83,640 s as one continuous measurement, in which a part of the 13,840 s relies on the Yb+ clock. Instability is gradually reduced with a slope of τ−0.37.

Combined instability of satellite links: Allan deviation of the difference between the frequency comparisons of UTC(NICT) and UTC(PTB) obtained via TWCP and GPSCP. Filled squares in black show the performance during the four days as the direct frequency comparison of two Sr87 clocks while filled circles in red represent the measurement of March and April in 2013.